Nonalternant Nanographenes Containing N-Centered Cyclopenta[ ef ]heptalene and Aza[7]Helicene Units.

Introducing helical subunits into negatively curved π-systems has a significant effect on both the molecular geometry and photophysical properties; however, the synthesis of these helical π-systems embedded with nonbenzenoid subunits remains challenging due to the high strain deriving from both the curvature and helix. Here, we report a family of nonalternant nanographenes containing a nitrogen (N)-doped cyclopenta[ ef ]heptalene unit. Among them, CPH-2 and CPH-3 can be viewed as hybrids of benzoannulated cyclopenta[ ef ]heptalene and aza[7]helicene. The crystal structures revealed a saddle geometry for CPH-1 , a saddle-helix hybrid for CPH-2, and a twist-helix hybrid for CPH-3 . Experimental measurements and theoretical calculations indicate that the saddle moieties in CPHs undergo flexible conformational changes at room temperature, while the aza[7]helicene subunit exhibits a dramatically increased racemization energy barrier (78.2 kcal mol -1 for CPH-2 , 143.2 kcal mol -1 for CPH-3 ). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta[ ef ]heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in CPH-1 and CPH-2 and the similar energy fluorescence and phosphorescence in CPH-3 . Both enantiopure CPH-2 and CPH-3 display distinct circular dichroism (CD) signals in the UV- vis range. Notably, compared to the reported fully π-extended helical nanographenes, CPH-3 exhibits excellent chiroptical properties with a | g abs | value of 1.0 × 10 -2 and a | g lum | value of 7.0 × 10 -3 ; these values are among the highest for helical nanographenes.